1. Field of the Invention
This invention relates to an electroluminescent device [EL device], particularly to an EL device having two independent luminescent layers, which can individually be controlled to control freely the tone of the luminescent color, intensity, etc. and simultaneously has a sufficient luminance.
2. Description of the Prior Art
EL device generally has a structure, comprising a luminescent layer containing a material having EL function, namely a material having the function of emitting light when placed in an electrical field, arranged between two electrodes, and it is a luminescent device which emits light by directly converting electric energy to light by applying voltage between these electrodes to generate an electrical field. It is different from a light emitting system of the prior art, for example, an incandescent lamp in which filament is heated to emit light or a fluorescent lighting in which an electrically excited gas imparts energy to a fluorescent material to effect emission, and it is attracting attention because of having the possibility of being realized as constituent member for lamps, displaying medium to be used for display of lines, figures and images, of various shapes such as thin panels, belts, cylinders, etc., or panel lamps, etc. of larger area.
As the material having EL function to be used for EL device, there have been used in the prior art primarily inorganic metal materials such as ZnS, etc. containing Mn, Cu or ReF.sub.3 (Re represents a rare-earth element) as the activator. Also, recently, as shown by the EL device employing anthracene, pyrene or perylene or derivatives thereof as the material for luminescent layer disclosed in Japanese Laid-open Patent Publication No. 172891/1983, organic compound materials capable of forming thin films with good precision according to various thin film forming techniques are attracting attention.
The color of the light emitted from such an EL device can be controlled primarily by combining suitably selected materials to be used for luminescent layer or changing the intensity of the electrical field.
However, when an EL device is formed by use of the materials capable of forming the luminescent layer, the color and intensity of luminescence are limited to some extent, and it has been most difficult to change greatly the luminescent color, control severely the tone or control freely the intensity of luminescence in one EL device.
On the other hand, when a so-called thin film type EL device is produced by formation of a thin film luminescent layer according to the thin film forming method such as vapor deposition by use of the above inorganic metal material comprising mainly ZnS, there is involved the problem that the production cost is very high, and also no EL device of good quality having a large area could be produced with good bulk productivity, because a luminescent layer of large area comprising a uniform thin film could be formed only with extreme difficulty.
In contrast, as an EL device enriched in bulk productivity and advantageous in cost, an organic powder type EL element of the intrinsic EL system, comprising a luminescent layer formed by dispersing the above EL inorganic material composed mainly of ZnS in an organic binder, has been known.
However, in this powder type El device, when the layer thickness is made thinner, defects such as pinholes, etc. will readily be formed in the luminescent layer and therefore the layer thickness of the luminescent layer can be made thinner to a limited extent in its structure for the purpose of enhancing sufficiently the luminescent characteristic, whereby no sufficient luminescence, particularly luminance can be obtained. Also, due to the relatively thick layer thickness, there is also involved the problem of greater power consumption for generation of stronger electrical field. For generation of strong electrical field within the luminescent layer possessed by such a powder type EL device, an improved type powder EL device provided with an intermediate dielectric layer comprising vinylidene fluoride copolymer has been known, as disclosed in Japanese Laid-open Patent Publication No. 172891/1983, but under the present situation, no satisfactory performance with respect to luminance, power consumption, etc. has yet been obtained.
Also, in the EL device employing an organic material such as anthracene, etc., although the luminescent layer can be formed as a thin film with good precision, the density of electrons or holes which are carriers is very small, whereby excitation probability of functional molecules through migration or recombination of carriers is so low as to give no efficient luminescence. Particularly, it is unsatisfactory in power consumption and luminance under the present situation.